This invention relates to blades that are particularly useful as wind turbine rotor blades and to rotors and wind turbines utilizing such blades.
At least one known rotor blade configuration includes an aft-swept rotor blade design. When a wind gust strikes a blade with aft sweep, an increase in out-of-plane (flapwise) loading produces a pitching moment about sections further inboard. This pitching moment acts to induce outer portions of the blade to twist the leading edge of the blade sections into the wind so as to reduce an aerodynamic angle of attack of those sections, thereby ameliorating peak transient loads that the blade would otherwise experience. However, if the root of the blade remains unswept or is swept aft, the pitching moment induced over the entire blade is reacted at the root of the blade through pitch drive hardware. For even modest sweep, this moment can overwhelm baseline aerodynamic pitching moments for which the pitch hardware is designed. In other words, although in-plane aft sweep of wind turbine rotor blades can be used to ameliorate transient loads, aft sweep also induces pitching moments at the blade root that can overwhelm the baseline aerodynamic pitching moments for which the pitch hardware is designed.
For example, many known straight swept blades with zero root sweep exhibit very high changes in root torsion due to coupling. The same is true of some curved blades designed without a constraint on root torsion. At least one known baseline configuration exhibits a maximum nose-down pitching moment about the blade root of approximately 30-40 kNm. In some swept blades, the nose-down moment increases to hundreds of kNm. Pitch drives on at least one known wind turbine model, the GE 1.5 turbine available from General Electric Co., Fairfield, Conn. are designed, with safety factors included, for 100 kNm applied load. Therefore, increases in nose-down moment of more than 20-30 kNm should be avoided.